The invention relates to an axial piston compressor with a drive shaft, a disc mounted on the drive shaft so that it can be tilted relative to the latter about a pivotal axis, and at least one piston provided with sliding blocks that move along a slideway on the disc.
Such an axial piston compressor can be used in particular in an air conditioner for motor vehicles. It serves to suck a coolant out of a heat-transfer compartment, in which the coolant evaporates while taking up heat, and to compress it to a higher pressure so that in another heat-transfer compartment the heat can be given off at a higher temperature level. Subsequently the coolant passes into an expansion organ, where it is returned to the pressure level of the first heat-transfer compartment.
For vehicle air conditioners coolant compressors of various constructions are employed. In recent years, for several reasons, axial piston compressors have come into general use, in particular because this construction enables an energetically favourable regulation of the output. That is, the compressor is customarily coupled directly to the motor by a belt drive, so that the operating conditions of the compressor cannot be adjusted as desired by changing the rotational speed of the compressor; for this reason the output is altered by tilting the disc, which determines the volume displaced by the compressor piston.
The stroke of each piston is produced by the cooperation between the sliding blocks connected to the piston and the disk, which can be pivoted relative to the drive shaft. When the disk is not tilted with respect to the drive shaft, i.e. the central axis of the disc coincides with the long axis of the drive shaft, there is no stroke, because the distance between, for example, the floor of the cylinder within which the piston is disposed and the bearing surface does not change when the drive shaft rotates. On the other hand, when the disc is tilted so that the angle between the central axis of the disc and the long axis of the drive shaft is different from zero, usually at most 20xc2x0, the distance between the bearing surface of the disc and the floor of the cylinder changes periodically between a minimal and a maximal value during each rotation of the drive shaft. Thus when the distance isminimal, the piston coupled to the disc is at its top-dead-centre position, i.e. is inserted maximally into the cylinder, whereas when the distance is maximal, the piston is at bottom dead centre.
The slideway, i.e. the path on the disc surface along which the sliding blocks mounted on the piston move, changes according to the angle at which the disc is tilted.
When the central axis of the disc coincides with the long axis of the drive shaft, the sliding blocks move over the disc along a circular slideway, the radius of which corresponds to the distance between the centre of the sliding blocks and the long axis of the drive shaft. In contrast, when the disc is tilted, the sliding blocks move along an elliptical slideway, because the distance between the middle of the sliding blocks and the long axis of the drive shaft is unchanged. The minor axis of the ellipse has a length corresponding to the radius of the circular slideway on a disc that is not tilted, and is parallel to the pivotal axis of the disc. The length of the major axis of the ellipse is equal to the radius divided by the cosine of the tilt angle of the disc.
To make the compressor compact, the pivotable disc is dimensioned so that when it is not tilted, there remains only a very small margin between the slideway of the sliding blocks and the outer edge of the disc. As a result, when the disc is tilted, the slideway overlaps the edge of the disc in the regions of the disc that correspond to the upper and the lower dead-centre points. This is a consequence of the apparent shortening of the disc when it is pivoted. Because of the fact that the slideway overlaps the edge when the disc is tilted, the area available to transfer the forces between disc and sliding blocks is reduced. Furthermore, in one of the positions in which the sliding blocks overlap the edge of the disc to the greatest extent, namely the position corresponding to the top-dead-centre point of the piston at the end of the compression stroke, the force exerted between the sliding blocks and the disc is maximal. Because the reduction of the area available for force transfer coincides with the maximum of the force to be transferred, the surface pressure between the disc and the sliding blocks increases, which in the extreme case can cause severe abrasion between these structures.
The objective of the invention is thus to improve an axial piston compressor of the kind described above in such a way that abrasion between the sliding blocks and the disc is reliably prevented under all operating conditions.
In an axial piston compressor in accordance with the invention, with the features cited in the characterizing part of claim 1, the pivotal axis of the disc is offset from the disc""s central plane; as a result, a translational movement is superimposed on the rotational movement of the disc. The consequence is that when the disc is pivoted, it shifts relative to the sliding blocks, the position of which is fixed. This shifting can be used to alter the amount by which the sliding-block slideway overlaps the edge of the disc to a specific degree, either markedly reducing the overlap or eliminating it entirely. This reduces or eliminates the increase in surface pressure between sliding block and slideway.
Preferably it is provided that the displacement of the pivotal axis of the disc from the mid-plane of the disc is towards the side of the disc that faces the piston. In this configuration the reduction of contact area between the sliding blocks and the disc brought about by tilting of the disc is counteracted in the region corresponding to the top-dead-centre point of the associated piston, i.e. at the operating point at which the force acting on the piston is greatest. The reduction of contact area between sliding block and edge of the disc that does occur in this configuration, which is twice as great as in a configuration according to the state of the art (with a pivotal axis that coincides with the mid-plane of the disc), can be tolerated because at the corresponding time the force acting on the piston is comparatively slight. Even though the contact area between sliding blocks and disc surface is reduced, the resulting surface pressure is below the critical values.
According to one preferred embodiment of the invention the disc is a swash plate, which can be set into rotation by the drive shaft and the tilt angle of which with respect to the drive shaft can be adjusted. Such an axial piston compressor, whichxe2x80x94apart from the translational movement that is superimposed on the rotational movement of the discxe2x80x94corresponds to the structure known for example from the patent DE 197 03 216 A1, combines the advantage obtained in accordance with the invention, namely a reduction of surface pressure at certain times during operation such as the time when the force acting on the piston is maximal, with the advantage of relatively simple construction that this kind of structure provides.
According to an alternative preferred embodiment it is provided that the disc is a wobble plate, which is rotatably mounted on a swash plate and the tilt angle of which with respect to the drive shaft corresponds to that of the swash plate. This kind of structure, whichxe2x80x94apart from the translational movement of the wobble plate that is superimposed on the rotational movement during pivotingxe2x80x94corresponds to a structure known for example from the patent DE 196 21 174 A1, combines the advantage of a targeted reduction of surface pressure with the advantage of particularly low-friction operation that this kind of structure provides.
According to a preferred embodiment of an axial piston compressor in accordance with the invention it is provided that with a distance of 30 mm between the long axis of the drive shaft and the long axis of the piston, an 8-mm diameter of the flat surfaces of the sliding blocks, which are apposed to the disc, and an angle of maximally 180 between the long axis of the drive shaft and the central axis of the disc, the distance between the mid-plane of the disc and the pivotal axis of the disc is about 1 mm. With this slight offset between pivotal axis and disc mid-plane, when the disc is tilted it is displaced relative to the slideway of the sliding blocks only far enough that on one side of the disc the degree to which the slideway overlaps the outer edge of the disc is reduced. Although it is theoretically possible to shift the disc so far that the slideway is confined entirely to the disc in the region of one dead-centre point of the piston, the invention is not intended to produce this effect; as the distance by which the pivotal axis is offset from the mid-plane of the disc increases, the centre of mass of the disc also moves away from the long axis of the drive shaft. The value given above, if the geometric relationships are as described, represents a good compromise between a reduction of surface pressure on one hand and an increased imbalance of the disc on the other hand.
Advantageous embodiments of the invention will be apparent from the subordinate claims.